Neighborhood Sunday drive-by honker: what’s the deal?

Every Sunday, somebody drives through my suburb neighborhood tapping their horn as they slow roll through. They tap horn a few times a a second. It's not terribly obnoxious since it's around noon, but they drive just fast enough that I can't get out the door and ask them what's going on.

Does anyone know the purpose of the slow rolling honkers? Are they independent scrap collectors trying to advertise /solicit for donations?